[sensors] PlatformSensorProvider implementations are notified when there are no sensor instances

device/generic_sensor/platform_sensor_provider_base.h

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef DEVICE_GENERIC_SENSOR_PLATFORM_SENSOR_PROVIDER_BASE_H_
 #define DEVICE_GENERIC_SENSOR_PLATFORM_SENSOR_PROVIDER_BASE_H_
 
 #include "base/macros.h"
 
 #include "base/threading/non_thread_safe.h"
@@ skipping 13 matching lines @@
   // Creates new instance of PlatformSensor.
   void CreateSensor(mojom::SensorType type,
                     const CreateSensorCallback& callback);
 
   // Gets previously created instance of PlatformSensor by sensor type |type|.
   scoped_refptr<PlatformSensor> GetSensor(mojom::SensorType type);
 
   // Shared buffer getters.
   mojo::ScopedSharedBufferHandle CloneSharedBufferHandle();
 
+  // Returns 'true' if some of sensor instances produced by this provider are
+  // alive; 'false' otherwise.
+  bool HasSensors() const;
+
  protected:
   PlatformSensorProviderBase();
   virtual ~PlatformSensorProviderBase();
 
   // Method that must be implemented by platform specific classes.
   virtual void CreateSensorInternal(mojom::SensorType type,
                                     mojo::ScopedSharedBufferMapping mapping,
                                     const CreateSensorCallback& callback) = 0;
 
+  // Implementations might override this method to free resources when there
+  // are no sensors left.
+  virtual void AllSensorsRemoved() {}
+
  private:
   friend class PlatformSensor;  // To call RemoveSensor();
 
   bool CreateSharedBufferIfNeeded();
   void RemoveSensor(mojom::SensorType type);
   void NotifySensorCreated(mojom::SensorType type,
                            scoped_refptr<PlatformSensor> sensor);
 
  private:
   using CallbackQueue = std::vector<CreateSensorCallback>;
 
   std::map<mojom::SensorType, PlatformSensor*> sensor_map_;
   std::map<mojom::SensorType, CallbackQueue> requests_map_;
   mojo::ScopedSharedBufferHandle shared_buffer_handle_;
 
   DISALLOW_COPY_AND_ASSIGN(PlatformSensorProviderBase);
 };
 
 }  // namespace device
 
 #endif  // DEVICE_GENERIC_SENSOR_PLATFORM_SENSOR_PROVIDER_BASE_H_